The present invention relates to a wire cut electric discharge machine which machines a workpiece by generating an electrical discharge between a travelling wire electrode and the workpiece. In particular, the present invention relates to a wire cutting device which in the wire electrode used to generate an electrical discharge is cut into small pieces.
Wire cut electric discharge machines in which the wire electrode (referred to below as xe2x80x9cwirexe2x80x9d) is conveyed from a wire supply bobbin through a tensioning device and/or multiple pulleys to a wire uptake roller or a wire pull-in roller are known. In such machines, the workpiece is machined by causing an electrical discharge in the space or xe2x80x9cgapxe2x80x9d formed between the wire, which is moving under a specified tension, and the conductive workpiece. Copper or brass wire having a diameter of 0.1 mm to 0.3 mm is generally used for the wire electrode. When necessary; molybdenum or tungsten wire, both of which have a high tensile strength, may be used when a diameter under 0.1 mm is desired. A wire pull-in roller pulls the wire used to generate an electrical discharge, and the spent wire is fed out to an appropriate wire collection container, such as a bucket. Such a wire cut electric discharge machines has a high machining accuracy and are appropriate for precision machining.
In recent years, in order to operate wire cut electric discharge machines automatically over many hours, large wire supply bobbins, such as 10 Kg rolls or 50 Kg rolls, have come into use. At the same time, the necessity has arisen for large wire collection containers capable of holding large amounts of spent wire. Japanese Utility Model 2-15825 and Japanese Patent 61-53170 disclose wire cutting devices which are placed between the wire take-up roller and the wire collection container, and which cut the used wire into small pieces. By means of such a wire cutting device, it is possible to hold a large volume of wire without resort to an excessively large capacity wire collection container.
The wire cutting device disclosed in Patent 61-53170 consists of a gear-shaped cutter on which cutting blades which cut the wire are equally spaced around the perimeter, and a roller on which protuberances used to tension the wire for cutting are provided around the perimeter at the same spacing as the cutting blades. The gear-shaped cutter and roller are placed so as to maintain a spacing determined by the wire diameter. Furthermore, the drive mechanism which rotates the gear-shaped cutter and roller in consonance with the wire feed direction is mounted on the wire cutting device. Used wire is fed between the gear-shaped cutter and the roller, and is sheared by the wear-free blade edge of the gear-shaped cutter blade, while being held in place in the gap between the cutting blade and the roller protuberances. The cutting characteristics of this device are greatly affected by the shape of the cutting blade edge. For example, blade edge wear can cause rounding, and wear characteristics of the wire caused by electrical discharge can change, leading to cutting defects. This leads to problems such as the need to frequently change the gear-shaped cutter, or to frequently precision-adjust the relative positions of the gear-shaped cutter and the roller.
An object of the present invention is to provide a wire cutting device for a wire cut electric discharge machine for which fine adjustments of the gear are unnecessary, even if there are changes in wire diameter, material, or wire wear condition.
A further object of the present invention is to provide a wire cutting device for a wire cut electric discharge machine which does not require fine adjustment of the gear even if the shape of the gear protuberances change due to wear so that the, cutting performance will not degrade even over extended time periods.
Other objects of the present invention are described in the explanation which follows, and will in part become apparent to practitioners of the art through implementation of the invention.
In order to achieve the above objectives, the wire cutting device for cutting a wire electrode used to generate an electrical discharge between a wire electrode and a workpiece comprises:
a drive gear;
a follower gear which engages the drive gear and which is freely profile shiftable relative to the drive gear;
a bias mechanism for pressing the follower gear against the drive gear;
a wire fake-up for feeding the spent wire electrode between the drive gear and the follower gear; and
a drive mechanism which drives the drive gear such that the perimeter speed of the drive gear is greater than the speed at which the wire electrode is fed out by the wire outfeed device.
As the drive gear and follower gear rotate, the wire electrode sandwiched between the gears is held in place between two adjacent teeth of one of either the drive gear or the follower gear; the portion of the wire electrode held in place is pressed into one of the gears by the teeth of the other gear, and is cut by tensile force.
Preferably, the wire cutting device will include a wire position adjustment device which, in order to change the contact position of the wire electrode and the gears, will either more of the gear or the wire electrode relative to the other in the direction of the gear rotational axis.
Further, the pressing force of the pressing force imparting mechanism is preferably adjustable.